NO DISSOLUTION FAULTING NEEDED - MESOZOIC FOLD COLLAPSE OF THE CRESTS OF THE PARADOX SALT WALLS

The Paradox Basin salt walls have been interpreted as diapirs that grew from the Permian through the Jurassic, followed by “burial” in the latest Jurassic-Cretaceous and “collapse” in the Neogene. Previous studies have favored Neogene collapse by either salt dissolution with subsidence on ring faults or graben formation on diapir crests during extension. Gypsum Valley diapir in the southern Paradox Basin indicates that the burial and collapse history and style varied along the salt wall and occurred in two phases. The first was during the Triassic-Jurassic, and was associated with the formation of suprasalt folds. Only parts of the diapir exhibit evidence of Neogene solution fault collapse related to exhumation of the Colorado Plateau.

Gypsum Valley salt wall is flanked by salt-withdrawal minibasins. Within these minibasins, Pennsylvanian and Permian strata steepen toward the diapir where they are erosionally truncated on the flanks of the salt wall. In contrast, overlying Triassic and Jurassic strata thin, but pass unfaulted onto the top of the diapir where they partially underlie roof synclines that are filled with Late Jurassic Morrison Fm. growth strata. Subsidence of the diapir to form a roof syncline represents a style of diapir collapse that occurred prior to the Neogene solution collapse on ring fractures. Triassic and Jurassic formations form mostly unfaulted anticlines that drape the diapir margin and extend beneath the roof synclines. Over the course of 65 million years, from the Middle Triassic to the Late Jurassic, strata gradually overlapped and buried the NW half of the diapir, reducing the length of the passively-rising salt wall from 24 km to 13 km. The Morrison-filled roof syncline fold collapse post-dated the partial burial of the diapir by 10 million years and represented a distinct event of roof subsidence which may have been due to either salt migration along strike to the SE, where the diapir was still growing, or an earlier phase of dissolution not related to Neogene fault collapse. Younger solution collapse fault features are superimposed on the Jurassic synclines and are restricted to the center of the diapir in its NW end. The SE half of the diapir continued to passively rise through the Mesozoic and exhibits more widespread Neogene dissolution faulting and collapse.